1. Field of the Invention
The present invention relates to absorbent structures. Specifically, the present invention relates to an absorbent structure having osmotically enhanced absorption characteristics.
2. Description of the Related Art
The use of water-swellable, water-insoluble polymeric materials as absorbents is known. For example, particles of water-swellable, water-insoluble polymeric materials have been incorporated into batts of wood pulp fluff to increase the absorptive capacity of the batts. Such batts are incorporated into articles such as diapers, sanitary napkins, adult incontinence products and the like. Such water-swellable, water-insoluble polymeric materials are often capable of imbibing many times their weight in water. For example, it is not unusual for one gram of water-swellable, water-insoluble polymeric material to be able to absorb more than 10 grams of a liquid. Thus, such water-swellable, water-insoluble polymeric materials are commonly, and will hereinafter be, referred to as superabsorbent materials, incorporation of such superabsorbent materials into absorbent batts significantly increases the absorptive capacity of the batts.
When such superabsorbent materials are incorporated into absorbent batts, which batts are placed in absorbent products such as diapers, it is also desirable that the superabsorbent materials be able to retain an absorbed liquid when a modest amount of pressure is applied to the superabsorbent material. Additionally, it is desired that the superabsorbent materials be able to imbibe a liquid in a rapid manner. Unfortunately, in the absence of geometric influences, the rate at which superabsorbent materials can imbibe a fluid is generally inversely related to the ability of the superabsorbent materials to swell. That is, the higher the equilibrium swelling of a superabsorbent material, the slower the rate of uptake.
Compared to the absorbent batts in which the superabsorbent materials may be incorporated, the superabsorbent materials are relatively expensive. Therefore, when it is desired to employ the superabsorbent materials in an absorbent product and minimize the ultimate cost of the absorbent product, it is desirable to minimize the amount of superabsorbent material present in the product while maintaining the performance of the product. Accordingly, several attempts have been made to optimize various liquid absorbing characteristics of known water-swellable polymers.
For example, Japanese Kokai Patent No. SHO 62[1987]-212404 published Sept. 8, 1987, is directed to the preparation of highly water-swellable polymers. The polymeric materials are prepared by emulsion polymerization of monomers known to form superabsorbent materials in the presence of monosaccharides and/or oligosaccharides and a crosslinking agent. Such a polymerization process is described as producing polymeric materials which are grafted onto the mono and/or oligosaccharides. The polymers are described as having an improved absorption rate; moreover, reference to Table 1 of the Japanese publication indicates that the absorption capacity is similarly improved.
U.S. Pat. No. 4,541,871, issued Sept. 17, 1985, to Obayashi et al. describes a two-step process for preparing crosslinked carboxylic acid salt-containing polymers. According to the process, a lightly or even non-crosslinked polymer is slightly swollen and then further crosslinked with a difunctional material reactive with carboxylate groups. This process is described as producing a polymer particle which is more highly crosslinked on the outer surface then on the interior thereof. The particles are described as being capable of more rapid wetting without clumping or gel blocking.
U.S. Pat. No. 4,783,510, issued Nov. 5, 1988, to Saotome is directed to a process for improving a water absorbent poly(acrylic acid) and an improved polymer produced by said process. The process involves contacting a water absorbent poly(acrylic acid) with an aqueous solution containing a water soluble peroxide radical initiator. This contacting step is followed by heating which causes substantially only the surface portion of the polymer to undergo crossling with the radical initiator while leaving the remaining core portion of the polymer substantially intact. The polymer so produced is described as being excellent in both water absorbency and water absorption rate.
U.S. Pat. No. 4,693,713 issued Sept. 15, 1987, to Chmelir et al. is directed to an absorbent for blood and serous body fluids. The absorbents comprise a mixture of a superabsorbent material and a water soluble compound such as methylurea, monosaccharides, inorganic acids, salts of mono or poly-carboxylic acids and the like. The presence of the second component is described as acceerating the capillary flow of blood through a mass of the particulate absorbent component, thereby increasing the rate of absorption of the mixture.
In a similar manner, but for a different purpose, U.S. Pat. No. 4,473,670 issued Sept. 25, 1984, to Kessidis is directed to a salt-filled absorbable polymer. The absorbable polymer described by Kessidis is one which is capable of being metabolized by a living organism. That is, the polymers are capable of being placed in a living organism and subsequently dissolved and absorbed by the organism. Kessidis describes the use of a finely divided filler of sodium chloride or potassium chloride in such absorbable polymers to increase the rate of absorption of such polymers. In use, the filler material is quickly dissolved creating more surface area for enzymatic attack and quicker decomposition of the polymer in a biological system.
Similarly, U.S. Pat. No. 3,121,427, issued Feb. 18, 1964, to Mosier describes a catamenial appliance. In connection with the catamenial appliance, Mosier describes a mixture of a gelling agent, such as gelatin and/or agar-agar and sugar. The mixture of gelling material and sugar is wrapped in a tissue material. Gelling of menses through the gelling materials interacting with the hemoglobin in blood is described as proceeding through osmotic forces. The water-soluble inert filler, sugar or sodium chloride, is described as continuously and rapidly dissolving, thereby exposing fresh surfaces of the gel-forming matrix.
U.S. Pat. No. 4,742,086, issued May 3, 1988, to Masamizu, et al. is directed to a process for manufacturing a porous polymer. Described are polymeric particles containing a plurality of voids in their interior. The particles are described as having a small apparent specific gravity and as being superior in terms of water absorbing properties, permeability, and elasticity.
U.S. Pat. No. 4,686,776 issued Aug. 18, 1987, to Matsubara is directed to a dehydrating device. The device comprises a high osmotic pressure substance, a polymeric water absorber, and a hydrophilic alcohol. The three substances are copresent and integrally covered with a semi-permeable membrane. Water can permeate through the membrane and be absorbed by the polymeric water absorber. The presence of the high osmotic pressure substance is described as increasing the rate of water transport through the semi-permeable membrane.
U.S. Pat. No. 3,419,345 issued Dec. 31, 1968, to Parrish is directed to the neutralization of hydrophilic gel fibers of a cellulose derivative. This reference describes a process by which swellable fibers of a cellulose derivative are neutralized with an aqueous, concentrated, buffered salt solution having a pH within the range of about 5.0 to about 9.0. In this instance the buffered salt solution is employed to act as a base to neutralize the acid functionality and is not described as being used to incorporate salt into the cellulose fibers.
In a similar fashion, but directed to oxidized cellulose, U.S. Pat. No. 3,551,410 issued Dec. 29, 1970, to MacDonald et al. describes increasing the water-retentivity of cellulose fibers and the product produced thereby. MacDonald et al describe that the ability of a carboxyl cellulose to retain water is increased by soaking the carboxyl cellulose in a buffered salt solution.
Several techniques to improve the absorption rate and capacity of the superabsorbent material through chemical means are described. Unfortunately, none of the above references describe or suggest simple means by which the absorptive capacity of a given superabsorbent material can be increased on a gram of absorbed liquid per gram of polymer basis beyond that capacity due to the superabsorbent material itself. The concept of utilizing geometric configurations to increase absorption rate and/or capacity is not described by the references.